1. Field of the Invention
The present invention relates to a power combiner, an amplifier and a transmitter which are applicable to, for example, the power combining.
2. Description of the Related Art
In general, a radio transmitter has a Power Amplifier (PA) to send a signal with a predetermined power to antenna. In case that the large power is particularly necessary, a plurality of amplifier connected in parallel may be provided and each amplified output is combined to obtain the prescribed power, because there is a limitation in voltage amplitude in accordance with a characteristic of a amplifying device such as a transistor (for example, see the reference, “I. Aoki, et., al., “Fully Integrated CMOS Power Amplifier Design Using the Distributed Active-Transformer Architecture”, IEEE JSSC, Vol. 37, No. 3, March 2002, pp. 371-383.”; the entire contents of which are incorporated herein). The reference describes an example of the power amplifier having four differential amplifiers and four transformers for power combining.
In the power amplifier described in the reference, since all of the secondary terminals (the output terminals) of the transformers to combine the amplified outputs are connected in series, the series line length at the secondary terminals side is long. Therefore, a loss caused by the series resistance and the substrate resistance is large and the available frequency bandwidth is narrow. Namely, the loss at the power combiner is large, and as a result, there is a problem that the output power, the efficiency and the available frequency bandwidth of the power amplifier using the power combiner are degraded.
As stated above, in the conventional power combiner, amplifier and transmitter, there is a problem that the output power, the efficiency and the available frequency bandwidth of the power combiner, amplifier and transmitter are deteriorated. The present invention is made to solve the problem as stated above, and an object thereof is to realize a power combiner having low loss and wide available bandwidth characteristics as well as provide an amplifier and a transmitter having large power output, high efficiency and wide available bandwidth characteristics.